This invention relates to a traction type elevator system.
A prior-art traction type elevator system will be described with reference to FIGS. 1 to 3.
In FIG. 1, numeral 1 designates a driving sheave, numeral 2 a deflector wheel, numeral 3 a main rope made of a wire rope which is extended over the sheave 1 and the deflector wheel 2, numeral 4 a case suspended from one end of the main rope 3, and numeral 5 a balance weight suspended from the other end of the main rope 3. In FIGS. 2 and 3, the same numerals as in FIG. 1 indicate identical or corresponding portions. In the prior-art elevator system of this type, as shown in FIG. 2, the main rope 3 is wound around the sheave 1 and the deflector wheel 2 by two turns, while as shown in FIG. 3, a rope race 6, formed in the socalled undercut groove in a sectional view thereof in which substantially rectangular recesses are joined to the bottoms of circular arcs, is formed along the outer peripheral surface of the sheave 1 so as to hold the main rope 3 in engagement with the rope race 6.
In recent years, saving energy and saving resources have also been sought after in elevators. For example, as described in the official gazette of Japanese Patent Application Laying-open No. 50-48646, miniaturizing a hoisting machine and reducing the weight of a cage have been major objects.
For realizing the above requirements, it is a precondition to enhance the traction. As is well known, the traction is expressed by e.sup..mu.k.sbsp.2.sup..theta.. Also, that the following condition as specified by equation (1) must be met in order for the elevator to operate safely: EQU e.sup..mu.k.sbsp.2.sup..theta. &gt;(T.sub.2 /T.sup.1 ( 1)
where
e: the base of the natural logarithm,
.mu.: the friction factor between the rope race of the driving sheave and the main rope, k.sub.2 a coefficient depending upon the shape of the rope race,
.theta.: the winding angle of the main rope round the driving sheave,
T.sub.1 : the tension of the main rope on the cage side,
T.sub.2 : the tension of the main rope on the balance weight side.
Thus, the traction can be enhanced by increasing the friction factor .mu., the coefficient k.sub.2 depending upon the shape of the rope race and the winding angle .theta.. However, the increase of the friction factor .mu. spoils lubricity, and the increase of the coefficient k.sub.2, which depends upon the shape of the rope race raises the surface pressure. Further, the increase of either or both of these variables is undesirable because the lifetimes of the main rope and the sheave shorten due to the wear etc. thereof. It has been, accordingly, considered, to wind the main rope, having hitherto been wound round the sheave by two turns, by three turns in order to increase the winding angle .theta. of the main rope round the sheave. Even this measure, however, has had the disadvantage that the lifetime of the main rope is shortened due to the increase in the number of times the main rope is bent when the sectional shape of the rope race of the sheave in the prior art is left intact.